Wideband transmission with high spectral efficiency and high mobility is desirable for future wireless communications. Promising techniques to achieve this goal include an orthogonal frequency-division multiplexing (OFDM) technique and a multiple-input and multiple-output (MIMO) technique.
Traditionally, the OFDM technique uses a plurality of closely-spaced orthogonal subcarriers to carry data. The data may be allocated on a plurality of parallel subchannels, one for each of the subcarriers. Each of the subcarriers may be modulated with a conventional modulation scheme, e.g., quadrature amplitude modulation, at a relatively low symbol rate. In addition, an inverse fast Fourier transform (IFFT) may be performed on OFDM symbols representing the data on a transmitter side, e.g., at a base station, and a fast Fourier transform (FFT) may be performed to recover the OFDM symbols on a receiver side, e.g., at a user terminal. In addition, the OFDM technique may be used together with the MIMO technique, which uses multiple antennas at both the base station and the user terminal, to improve system performance.
Traditionally, a preamble signal including a preamble code may be used for timing synchronization of signals transmitted between a base station and a user terminal. For example, during a ranging process in which a timing offset between the base station and the user terminal is to be estimated, the base station may provide a plurality of ranging preamble codes (RPCs). The user terminal may randomly select one of the RPCs to generate a ranging preamble signal and transmit the ranging preamble signal to the base station. The base station may then estimate a timing offset between the base station and the user terminal, by estimating a signal round-trip delay (RTD) between the base station and the user terminal. The signal RTD is a time difference between a first time when the base station broadcasts the message and a second time when the base station receives the ranging preamble signal from the user terminal.
For example, based on the IEEE 802.16m standard, the base station may provide an RPC using a Zadoff-Chu sequence, as follows:
                                                        x              p                        ⁡                          (              k              )                                =                      exp            ⁢                          {                                                -                  j                                ·                π                ·                                                                                                    r                        p                                            ·                      k                      ·                                              (                                                  k                          +                          1                                                )                                                              +                                          2                      ·                      k                      ·                                              s                        p                                            ·                                              N                        CS                                                                                                  N                    RP                                                              }                                      ,                                  ⁢                  k          =          0                ,        1        ,        …        ⁢                                  ,                              N            RP                    -          1                ,                            equation        ⁢                                  ⁢                  (          1          )                    where:xp(k) is the Zadoff-Chu sequence;NRP is a length of the Zadoff-Chu sequence and has a value predetermined by the base station;p is an index of the Zadoff-Chu sequence and has a plurality of values predetermined by the base station;rp is a root parameter of the Zadoff-Chu sequence and has a plurality of values predetermined by the base station;sp is an index of each subcode of the Zadoff-Chu sequence and indicates an Spth cyclic shift version of the Zadoff-Chu sequence, and also has a plurality of values predetermined by the base station; andNcs is a cyclic shift parameter of the Zadoff-Chu sequence and has a value predetermined by the base station.
In equation (1), the item rp·k·(k+1) is also known as a root sequence part, and the item 2·k·sp·Ncs is also known as a cyclic shift part. Different values of the root parameter rp may generate different RPCs. For a given value of the root parameter rp, different values of the index sp may further generate subcodes for an RPC.
Traditionally, the base station may provide a plurality of RPCs by, e.g., broadcasting a message including the predetermined values of those parameters in equation (1), and the user terminal may randomly select one of the RPCs by selecting from the broadcasted message a value for each of those parameters and generating an RPC based on equation (1).
Based on the IEEE 802.16m standard, the user terminal may further generate a ranging preamble signal as follows:
                                          S            ⁡                          (              t              )                                =                      Re            ⁢                          {                                                ⅇ                                      ⅈ2π                    ⁢                                                                                  ⁢                                          f                      c                                        ⁢                    t                                                  ⁢                                                      ∑                                          k                      =                                                                        -                                                      (                                                                                          N                                RP                                                            -                              1                                                        )                                                                          /                        2                                                                                                            (                                                                              N                            PR                                                    -                          1                                                )                                            /                      2                                                        ⁢                                                                                    x                        p                                            ⁡                                              (                                                  k                          +                                                                                    (                                                                                                N                                  RP                                                                -                                1                                                            )                                                        /                            2                                                                          )                                                              ·                                          ⅇ                                              j                        ⁢                                                                                                  ⁢                        2                        ⁢                                                  π                          ⁡                                                      (                                                          k                              +                                                              K                                offset                                                                                      )                                                                          ⁢                        Δ                        ⁢                                                                                                  ⁢                                                                              f                            RP                                                    ⁡                                                      (                                                          t                              -                                                              T                                RCP                                                                                      )                                                                                                                                                          }                                      ,                            equation        ⁢                                  ⁢                  (          2          )                    where:S(t) is the generated preamble signal;t is an elapse time since the beginning of a current ranging process;fc is a carrier frequency;Koffset is a parameter relating to a frequency position;ΔfRP is a ranging subcarrier spacing; andTRCP is a duration of ranging cyclic prefix.
In reality, the base station provides a limited number of RPCs. When the base station simultaneously performs ranging with a plurality of user terminals, the provided RPCs may not be sufficient for each user terminal to select a different RPC, which may cause collision in the estimation of RTDs by the base station for the user terminals. Specifically, a size of the cell covered by the base station, communication channel conditions, and mobility of the user terminals may affect accuracy of the estimation of RTDs. As a result, system performance may be degraded.
For example, for the base station to cover a larger cell, a longer symbol duration is desirable. However, for a longer symbol duration, channel variations in time may introduce signal interference that decreases subchannel orthogonality, known as inter-carrier interference (ICI). The ICI may become severe as mobility of the user terminals, the carrier frequency, or the symbol duration increase, which may cause inaccuracy in the estimation of RTDs and therefore degrade system performance.
FIG. 1 shows a power delay profile (PDP) 100 obtained by a traditional base station covering a large cell during a ranging process. Due to the cell having a large size, the base station provides RPCs without applying cyclic shift, i.e., the cyclic shift part in equation (1) has a fixed value such as zero. When a first user terminal relatively close to the base station and a second user terminal relatively far away from the base station select a same RPC by selecting a same value for the root parameter to perform ranging, the base station may detect ranging preamble signals 102 and 104 from the first and second user terminals, respectively. However, because the ranging preamble signals 102 and 104 include the same RPC, the base station may not differentiate which ranging preamble signal is transmitted from which user terminal. As a result, a collision event occurs.
FIG. 2 shows a PDP 200 obtained by a traditional base station communicating with a first user terminal moving at a high speed and a second user terminal moving at a low speed during a ranging process. When the first user terminal and the second user terminal select different subcodes, e.g., Subcode 1 and Subcode 2, of a same RPC to perform ranging, the base station may detect ranging preamble signals 202 and 204 from the first and second user terminals, respectively. However, due to Doppler effects, the base station may also detect ICIs 212 and 214 of the ranging preamble signal 202 in time periods corresponding to Subcode 0 and Subcode 2 of the RPC, respectively. As a result, the base station may not differentiate which of the ranging preamble signal 204 and the ICI 214 is transmitted from the second user terminal, and a collision event occurs. In addition, the base station may interpret the ICI 212 detected in the time period corresponding to Subcode 0 as a ranging preamble signal from a third user terminal. As a result, a false alarm event occurs.